The present invention concerns a crystal oscillator configuration, the functional portions of which consist of individual 50 ohm subcircuits, and whose overall component count and circuit complexity is minimized.
In conventional low noise crystal oscillator circuits, the oscillator sustaining (gain) stage consists of one or more discrete transistors having low emitter impedance and moderate to high base and collector impedances referred to circuit ground. These impedances are usually substantially reactive, due to device junction capacitance and lead inductance. Also, in order to achieve adequate device gain, the transistors that are used must exhibit gain-bandwidth products that are well in excess of the oscillator operating frequency.
For example, a 100 megahertz oscillator circuit typically uses transistors having gain-bandwidth products of 1-3 gigahertz. The resulting circuit has several undesireable features, namely:
(1) The external circuitry to which the transistors are connected, including unavoidable printed circuit board track capacitance and inductance, result in marginal stability and sometimes instability (oscillation) of the device at frequencies above the desired operating frequency. Furthermore, this instability may be dependent on the operating temperature of the oscillator circuit, unit to unit component tolerances, or the manufacturers selected to provide the components used in the oscillator circuit. Moreover, such instabilities are not always observable in the engineering prototype circuits.
(2) Component screening for undesireable RF performance prior to circuit assembly is often inadequate, since the screening process often utilizes 50 ohm test equipment that does not adequately simulate the actual interface impedances.
(3) Additional components are often required in such typical oscillator circuits in order to suppress unwanted device instabilities and to match the impedance of the crystal and the other oscillator functional subcircuits to that of the discrete transistors. The resultant large component count and the usual use of variable or adjustable components further reduces the reliability of the oscillator circuit.